如何检查二叉搜索树是否完全平衡？

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def is_balanced(root):
    def check_balance(node):
        if node is None:
            return 0, True
        
        left_height, left_balanced = check_balance(node.left)
        right_height, right_balanced = check_balance(node.right)
        
        height = max(left_height, right_height) + 1
        balanced = left_balanced and right_balanced and abs(left_height - right_height) <= 1
        
        return height, balanced
    
    _, balanced = check_balance(root)
    return balanced